A conventional electroluminescent sheet element, as shown in a schematic cross-sectional view of FIG. 3, includes a fluorescent layer 1, used as a light emitting layer, sandwitched by insulating layers 2--2, with an AC voltage applied between a metal electrode 3 and a transparent electrode 4, so as to emit light from the fluorescent layer 1. Reference numeral 5 denotes a glass substrate.
The light emitting theory of the above electroluminescent sheet element is generally explained as follows, using an energy band schematic view of FIG. 4.
Electrons are discharged from the level of an interface between the fluorescent layer and the insulating layer or its neighborhood in the cathode's side to the transmission band of the fluorescent layer due to a tunnel effect. These electrons are accelerated in receipt of an energy from the electric field. At this time, the electrons excite the grating, and multiplication of electrons also occurs. Further, the electrons hit the light emission centers (for example, MN.sup.2+ ions) of the fluorescent layer and excite them. When the light emission centers L are returned from the excited state to the basic state, light is emitted. After this, the electrons are caught by the level of the interface between the fluorescent layer and the insulating layer in the anode's side. This is an alternation of the anode and the cathode, and it is repeated in sequence.
In the aforegoing light emitting theory, it is understood that the number of discharged electrons from the level of the interface between the insulator and the fluorescent layer to the transmission band of the fluorescent layer is determined by the density and the energy distribution, etc. of the interface level and that the interface level density and the energy distribution, etc. depend on the materials, crystal properties, layer-making methods, etc. of insulating and fluorescent layers. However, it is not yet possible to make the electroluminescent element under controls of the interface level density, energy distribution, etc.
Therefore, taking an electroluminescent sheet element using ZnS:Mn fluorescent layer, for example, as far as the same ZnS fluorescent layer manufacturing condition is used, various dielectric layers such as Y.sub.2 O.sub.3, SiO.sub.2, SiN.sub.4, Al.sub.2 O.sub.3, etc. as insulating layers does not provide large differences in the brightness and in the amount of moving electric charges under the same electric field intensity in the fluorescent layer. Its reason would be that varieties of materials of the insulating layers do not make large differences in the density and the distribution of the interface level and that since ZnS fluorescent layers formed under the same manufacturing condition are used, multiplication and scattering of electrons are in the substantially same degrees.
In this conventional arrangement, an increase in the amount of the moving electric charges caused by an increase of the number of injected electrons is not expected under the same electric field of the fluorescent layer, and an increase in the brightness is not expected either.